In multiuser uplink communication, multiple users communicate at the same time with an access point. To increase system capacity and facilitate a more efficient scheduling of users, these users can be separated in space (spatial division multiple access, SDMA) rather than be given a separate time slot (time division multiple access, TDMA) or frequency band (frequency division multiple access, FDMA). By employing multiple antennas at the access point and/or terminals, multiple users can transmit one or more data stream simultaneously, which are separated by multiple-input and multiple-output (MIMO) decoding at the access point. One problem is that each user may have a small carrier frequency offset (CFO) due to variations in the local oscillators, which needs to be estimated and corrected for at the access point.
In Orthogonal frequency-division multiplexing (OFDM) systems, a preamble is usually used to estimate the channel and any CFO. Since there are multiple CFOs (one for each terminal), the training sequence in the preamble must be designed accordingly. However, any residual CFO after estimation and compensation will cause phase rotations to the data symbols in the remaining part of the packet.